1. Field of the Invention
The present invention relates to a structure of a semiconductor device. More particularly, the present invention relates to a structure of complementary metal-oxide semiconductor (CMOS) sensor.
2. Description of the Related Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting light into an electrical signal that represents the intensity of the energy. The applications of the CCDs include monitors, transcription machines and cameras. Although CCDs have many strengths, CCDs also suffer from high costs and the limitation of the CCDs' volume. To overcome the weaknesses of CCDs and reduce costs and dimension, a CMOS photodiode device was developed. Because a CMOS photodiode device can be produced using conventional techniques, costs and the volume of the sensor can be reduced. The applications of CMOS photodiodes include PC cameras, digital cameras etc.
The photodiode is based on the theory that a P-N junction can convert light into an electrical signal. Before energy in the form of photons strikes the photodiode, there is an electric field in the P-N junction. The electrons in the N region do not diffuse forward to P region and the holes in the P region do not diffuse forward to N region. When enough light strikes the photodiode, the light creates a number of electron-hole pairs. The electrons and the holes diffuse forward to the P-N junction. While the electrons and the holes reach the P-N junction as a result of the effect of the inner electric field across the junction, the electrons flow to the N region and the holes flow to the P region. Thus a current is induced between the P-N junction electrodes. Ideally, a photodiode in the dark is open-circuit. In other words there is no current induced by light while the photodiode is in the dark.
FIG. 1A is a circuit diagram of a CMOS sensor. FIG. 1B is a layout of the sensor cell 110 in the FIG. 1A. FIG. 1C is a schematic, cross-sectional view of conventional CMOS sensor as taken along the I--I line in FIG. 1B.
As shown in FIG. 1A, the sensor array used in the latest CMOS sensor is improved from a passive pixel sensor array to an active pixel sensor array. The CMOS having the active pixel sensor array cell includes at least three active transistors 104, 106, 108 and a photodiode 102. The three active transistors are reset transistor 104, sense transistor 106 and select transistor 108. One of the source/drain regions of the transistor 104 is electrically coupled to the source voltage V.sub.DD. One of the source/drain regions of the transistor 106 is electrically coupled to the source voltage V.sub.DD. One of the source/drain regions of the transistor 108 is electrically coupled to the output. The sensor cell 110 comprises the transistor 104 and the photodiode 102. The photodiode 102 can convert light into an electrical signal by using the P-N junction and the electrical signal is transferred to the transistor 104.
As shown in FIG. 1B, the sensor cell 110 comprises the transistor 104 and the photodiode 102. The transistor 104 comprises a gate structure 104a, a source/drain region 104b adjacent to the gate structure 104a in the substrate. The sensor region 102a of the photodiode 102 is adjacent to the source/drain region 118 in the substrate.
As shown in FIG. 1C, the method of manufacturing the sensor cell 110 comprises providing a substrate 100 having an isolation region 112, an insulating layer 114 and a gate 104a. The insulating layer 114 can be a field oxide layer, for example. An ion implantation step is used to formed lightly doped drain (LDD) regions in portions of the substrate 100 exposed by the gate 104a and the isolation region 112. A spacer 116 is formed on the sidewall of the gate 104a. An ion implantation step is used to form heavily doped regions in portions of the substrate 100 exposed by the gate 104a, the spacer 116 and the isolation region 112. A source/drain region 104b is formed by a composition of the heavily doped region and the lightly doped drain region. A patterned photoresist (not shown) is formed over the substrate 100 to expose the region for the subsequently formed sensor region 102a. An implantation step with low energy and a high implanting dosage is performed to form a sensor region 102a across a portion of the source/drain region 118 and extending from the surface of the substrate 100 into the substrate 100.
Since the bird's beak region 112a is present at the boundary between the sensor region 112 and the sensor region 102a, the stress of the interface between the isolation region 112 and sensor region 102a is large. Because of the large stress, many crystal defects are present at the boundary between the sensor region 112 and the sensor region 102a. Therefore, the crystal defects induce large junction leakage current and dark current of the sensor. Furthermore, spots of light easily occur in the display image.
In order to overcome the problems induced by the bird's beak 112a, another conventional method of manufacturing a CMOS sensor was developed.
FIG. 2A is a layout of a sensor cell produced by another conventional method. FIG. 2B is a schematic, cross-sectional view of the conventional CMOS sensor referred to the II--II line in FIG. 2A.
Referring to FIG. 2A together with FIG. 2B, a gate 204a of a reset transistor 204 is formed on a substrate 200. A dummy shield layer 218 is formed on a isolation region 212 and covers the bird's beak region 212a. The gate 204a and the dummy shield layer 218 are formed in the same step. The region which is covered by the dummy shield layer 218 extends from the bird's beak region 212a extending 0.5 .mu.m to the reset transistor 204 and to the isolation region 212.
Because of the dummy shield layer 218, the subsequently formed sensor region 202a and the bird's beak region 212a are staggered. Therefore, the junction leakage current is small. Since the dummy shield layer 218 extends about 0.5 .mu.m form the bird's beak region 212a to the reset transistor 204, the size of subsequently formed sensor region 202a is limited. Furthermore, the efficiency and the effect of the sensor are poor.